These studies are designed to evaluate problems of platelet support including platelet collection, storage, indications for platelet transfusion and to identify and resolve the causes of ineffective platelet transfusions. Platelets collected by a continuous flow centrifuge will be cryopreserved and their subsequent viability and function determined after transfusion to thrombocytopenic patients. Stool blood loss and bleeding time measurements will be used to identify medications associated with an increased risk of bleeding in thrombocytopenic patients. HL-A typing and platelet crossmatching tests will be performed to evalutate methods which can be used to select compatible platelet donors for the thrombocytopenic, alloimmunized recipient. The causes and treatment of increased platelet consumption associated with malignancy will be investigated in both patients and in dogs. Whether DL-A matched sibling donor platelet transfusions prior to bone marrow transplantation prevent engraftment will be studied in a dog model.